The present invention relates to a plasma display device employing a plasma display panel (hereinafter referred to as a PDP), and in particular to a technology useful for increasing luminous efficiency.
Recently, plasma display devices employing an AC surface-discharge PDP are beginning to be mass-produced as a large-screen thin color display devices.
Presently, AC surface-discharge PDPs having a three-electrode structure as shown in FIG. 13 are widely used. In the AC surface-discharge PDP of FIG. 13, a discharge space 33 is formed between a pair of opposing glass base plates, a front base plate 21 and a rear base plate 28. The discharge space 33 is filled with a discharge gas (usually a mixture of gases such as He, Ne, Xe, Ar and others) at several hundreds or more of Torrs.
A plurality of pairs of X and Y electrodes for sustain discharge are disposed on the underside of the front base plate 21 serving as a display screen, for sustain discharge mainly for light emission for forming a display.
Usually, each of the X and Y electrodes is made of a combination of a transparent electrode and an opaque electrode to supplement conductivity of the transparent electrode.
The X electrodes are comprised of transparent X electrodes 22-1, 22-2, . . . and corresponding opaque X bus electrodes 24-1, 24-2, . . . , respectively, and the Y electrodes are comprised of transparent Y electrodes 23-1, 23-2, . . . and corresponding opaque Y bus electrodes 25-1, 25-2, . . . , respectively. It is often that the X electrodes are used as a common electrode and the Y electrodes are used as independent electrodes.
A discharge gap Ldg between the X and Y electrodes in one discharge cell are designed to be small such that a discharge breakdown voltage is not excessively high, and a spacing Lng between two adjacent cells is designed to be large such that unwanted discharge is prevented from occurring between two adjacent cells.
The discharge sustain X and Y electrodes are covered with a front dielectric substance 26 which, in turn, is covered with a protective film 27 made of material such as magnesium oxide (MgO).
The MgO protects the front dielectric substance 26 and lowers a discharge breakdown voltage because of its low sputtering yield and high secondary electron emission coefficient.
Address electrodes 29 (hereinafter referred to merely as an A-electrode) for addressing cells are disposed on the upper surface of the rear base plate 28 in a direction perpendicularly to the discharge sustain X and Y electrodes.
The address electrodes 29 are covered with a rear dielectric substance 30, separation walls 31 are disposed between the A-electrodes on the rear dielectric substance 30.
A phosphor 32 is coated in a cavity formed by the surfaces of the separation walls 31 and the upper surface of the rear dielectric substance 30.
In this configuration, an intersection of a pair of discharge sustain electrodes with an A-electrode corresponds to one discharge cell, and the discharge cells are arranged in a two-dimensional fashion.
In a color PDP, a trio of three discharge cells coated with red, green and blue phosphors, respectively, forms one pixel.
FIG. 14 and FIG. 15 are cross-sectional views of one discharge cell of FIG. 13 viewed in the directions of the arrows D1 and D2, respectively. In FIG. 15, the boundary of the cell is approximately represented by broken lines.
Now operation of the PDP will be explained.
The principle of generation of light by the PDP is such that discharge is started by a pulse applied between the X and Y electrodes, and ultraviolet rays generated by excited discharge gases are converted into visible light by the phosphor.
As shown in a block diagram of FIG. 16, the PDP 100 is incorporated into a plasma display device 102.
In FIG. 16, a driving circuit 101 receives signals for a display image from a video signal source 103, converts the signals into driving voltages as shown in FIGS. 17A to 17C, and then supplies them to respective electrodes of the PDP 100.
FIG. 17A is a time chart illustrating a driving voltage during one TV field required for displaying one picture on the PDP shown in FIG. 13. Portion of FIG. 17A illustrates that one TV field 40 is divided into sub-fields 41 to 48 having different numbers of light emission more than one from one another. Gray scales are generated by a combination of one or more selected from among the eight sub-fields.
Suppose eight sub-fields are provided which have gray scale brightness steps in binary number step increments, then each discharge cell of a three-primary color display device provides 28 (=256) gray scales, and as a result the three-primary color display device is capable of displaying about 16.78 millions of different colors.
Portion II of FIG. 17A illustrates that each sub-field comprises a reset discharge period 49 for resetting a discharge cell to an initial state, an address period 50 for addressing a discharge cell to be made luminescent, and a light-emission period (also called a discharge sustain period) 51.
FIG. 17B illustrates waveforms of voltages applied to the A-electrode 29, the X electrode and the Y electrode during the address period 50 shown in FIG. 17A. A waveform 52 represent a voltage V0 applied to one of the A-electrodes 29, a wave form 53 represent a voltage V1 applied to the X electrode, and waveforms 54 and 55 represent voltages V21 and 22 applied to ith and (i+1)st Y electrodes.
As shown in FIG. 17B, when a scan pulse 56 is applied to the ith Y electrode, in a cell located at an intersection of the ith Y electrode with the A-electrode 29 supplied with the voltage V0, first an address discharge occurs between the Y electrode and the A-electrode, and then an address discharge occurs between the Y electrode and the X electrode.
No address discharges occur at cells located at intersections of the X and Y electrodes with the A-electrode at ground potential.
The above applies to a case where a scan pulse 27 is applied to the (i+1)st Y electrode.
In the cell where the address discharges have occurred, charges (wall discharges) are generated on the surface of the dielectric substance 26 and the protective film 27 covering the X and Y electrodes by the discharges, and consequently, a wall voltage Vw (V) occurs between the X and Y electrodes as shown in FIG. 15.
In FIG. 15, reference numeral 3 denotes electrons, 4 is a positive ion, 5 is a positive wall charge, and 6 are negative wall charges.
The presence and absence of the wall charges corresponds to the presence and absence of sustain discharge during the succeeding light-emission period 51, respectively.
FIG. 17C illustrates pulse driving voltages (or voltage pulses) applied to the X and Y electrodes serving to sustain discharge and a driving voltage applied to the A-electrode, all at the same time during the light-emission period 51 shown in FIG. 17A.
The Y electrode is supplied with a pulse driving voltage of waveform 58, the X electrode is supplied with a pulse driving voltage of waveform 59, the magnitude of the voltages of the waveforms 58 and 59 being V3(V).
The A-electrode 29 is supplied with a driving voltage of waveform 60 which is kept at a constant voltage V4 during the light-emission period 51. The voltage V4 may be ground potential.
The pulse driving voltage of the magnitude V3 is applied alternately to the X electrode and the Y electrode, and as a result reversal of the polarity of the voltage between the X and Y electrodes is repeated.
The magnitude V3 is selected such that the presence and absence of the wall voltage generated by the address discharge correspond to the presence and absence of the sustaining discharge, respectively.
In the discharge cell where the address discharge has occurred, discharge is started by the first voltage pulse, and continues until wall charges of the opposite polarity accumulate to some extent.
The wall voltage accumulated due to this discharge serves to reinforce the second inverted voltage pulse, and then discharge is started again.
The above is repeated by the third and succeeding pulses.
In this way, in the discharge cell where the address discharge has occurred, sustain discharges occur between the X and Y electrodes the number of times equal to the number of the applied voltage pulses and emit light. On the other hand, the discharge cells do not emit light where the address discharge has not occurred.
At present, efficiency of luminescence of the PDP is inferior to that of a cathode ray tube, and therefore improvement of the efficiency of the PDP is necessary so that the PDPs spread as TV receivers.
There is also a problem in that, in realization of a large-screen PDP, a current to be supplied to its electrodes increases excessively and the power consumption increases.
When the size of the cell is reduced in order to increase the number of pixels and thereby increase the degree of definition of a display image, there is also a problem in that the efficiency of luminescence is reduced because of the reduction of the discharge space.
The improvement of luminous efficiency of the PDP is essential for solving the above problems.
Conventional techniques for improving the luminous efficiency include improvements of cell structures and driving methods.
For the improvement of cell structures, the improvements on the size or the shape of discharge sustain electrodes are disclosed in Japanese Patent Application Laid-open Nos. Hei 8-22772, Hei 3-187125, and Hei 8-315735. The improvements on material of the dielectric substance covering the discharge sustain electrode are disclosed in Japanese Patent Application Laid-open Nos. Hei 7262930 and Hei 8-315734. Some of the above have been put to practical use, but the luminous efficiency of the PDP is still inferior to that of a cathode ray tube.
For the improvement of a driving method, a method using a high frequency discharge is disclosed in IDW 1999 (Proceedings of the Sixth International Display Workshops), p. 691, but the day is still far off when this method can be put to practical use because of great dimensions of a required high frequency power source.
As described above, in the currently dominant three-electrode AC surface-discharge PDP, cell structures and driving methods have been improved for increasing the luminous efficiency.
There have been problems in that some of the above suggested improvements on the cell structures have been put to practical use, but the efficiency of luminescence of the PDP is still inferior to that of a cathode ray tube, and in that the improvement on the driving method by using a high frequency discharge has a difficulty in putting it to practical use because of the great dimensions of a required high frequency power source.
The present invention has been made to solve the above problems with the prior art, and it is an object of the present invention to provide a technology capable of improving the efficiency of sustain discharge in a plasma display device employing a plasma display panel by improving a driving method without the need for a huge high-frequency power source or the like.
The above and other objects and novel features of the present invention will be apparent from the description and the accompanying drawings.
The following explains briefly the summary of the representative ones of the present inventions disclosed in this specification:
In accordance with an embodiment of the present invention there is provided a plasma display device comprising: a plasma display panel having a pair of opposing base plates and a plurality of discharge cells formed between the pair of opposing base plates, each of the plurality of discharge cells having a pair of discharge sustain electrodes disposed on one of the pair of opposing base plates and an address electrode disposed on another of the pair of opposing base plates; and a driving circuit for driving the plurality of discharge cells, the driving circuit being configured such that at least one of the pair of discharge sustain electrodes is supplied with a pulse driving voltage within a period of light emission of a corresponding one of the plurality of discharge cells, an address electrode of at least one of the plurality of discharge cells is supplied with a driving voltage within the period of light emission, the driving voltage having a waveform including a portion varying to a voltage level Va in synchronism with variation from a first voltage level to a second voltage level of the pulse driving voltage and then varying to a voltage level Vb before the pulse driving voltage varies from the second voltage level to the first voltage level, an absolute value of the voltage level Vb not being greater than an absolute value of half the voltage level Va.
In accordance with another embodiment of the present invention, there is provided a plasma display device comprising: a plasma display panel having a pair of opposing base plates and a plurality of discharge cells formed between the pair of opposing base plates. Each of the plurality of discharge cells has a pair of discharge sustain electrodes disposed on one of the pair of opposing base plates and an address electrode disposed on another of the pair of opposing base plates; an inductance element connectable in series with the address electrode; and a driving circuit for driving the plurality of discharge cells, the driving circuit being configured such that at least one of the pair of discharge sustain electrodes is supplied with a pulse driving voltage within a period of light emission of a corresponding one of the plurality of discharge cells.
In accordance with another embodiment of the present invention, there is provided a plasma display device comprising: a plasma display panel having a pair of opposing base plates and a plurality of discharge cells formed between the pair of opposing base plates, each of the plurality of discharge cells having a pair of discharge sustain electrodes disposed on one of the pair of opposing base plates and an address electrode disposed on another of the pair of opposing base plates; a driving circuit for driving the plurality of discharge cells, the driving circuit being configured such that at least one of the pair of discharge sustain electrodes is supplied with a pulse driving voltage within a period of light emission of a corresponding one of the plurality of discharge cells; and a waveform generator for supplying to the address electrode a voltage varying in synchronism with the pulse driving voltage during at least a portion of the period of light emission.